[Telemetry] Fix race in session restore measurement.

tools/perf/measurements/session_restore.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
+
 import collections
 
 from measurements import startup
 from metrics import cpu
+from metrics import histogram_util
 from metrics import startup_metric
 from telemetry.core import util
 
+
 class SessionRestore(startup.Startup):
   """Performs a measurement of Chromium's Session restore performance.
 
   This test is meant to be run against a generated profile.
   This test inherits support for the --warm or --cold command line options -
   see startup.py for details.
   """
 
   def __init__(self, action_name_to_run = ''):
     super(SessionRestore, self).__init__(action_name_to_run=action_name_to_run)
     self.close_tabs_before_run = False
     self._cpu_metric = None
 
   def CustomizeBrowserOptions(self, options):
     super(SessionRestore, self).CustomizeBrowserOptions(options)
+    histogram_util.CustomizeBrowserOptions(options)
     options.AppendExtraBrowserArgs([
         '--restore-last-session'
     ])
 
   def TabForPage(self, page, browser):
     # Detect that the session restore has completed.
-    util.WaitFor(lambda: len(browser.tabs), 30)
-    return browser.tabs[0]
+    util.WaitFor(lambda: browser.tabs and
+                 histogram_util.GetHistogramCount(
+                     histogram_util.BROWSER_HISTOGRAM,
+                     'SessionRestore.AllTabsLoaded',
+                     browser.foreground_tab),

[jeremy, 2014/06/05 10:39:04]

Very nice!!!

+                 30)
+    return browser.foreground_tab
 
   def CanRunForPage(self, page):
     # No matter how many pages in the pageset, just perform one test iteration.
     return page.page_set.pages.index(page) == 0
 
   def RunNavigateSteps(self, page, tab):
     # Overriden so that no page navigation occurs.
     pass
 
   def ValidatePageSet(self, page_set):
     wpr_archive_names_to_page_urls = collections.defaultdict(list)
     # Construct the map from pages' wpr archive names to pages' urls.
     for page in page_set:
       if page.is_local:
         continue
       wpr_archive_name = page_set.WprFilePathForPage(page)
       wpr_archive_names_to_page_urls[wpr_archive_name].append(page.url)
 
     # Reject any pageset that contains more than one WPR archive.
     if len(wpr_archive_names_to_page_urls.keys()) > 1:
       raise Exception("Invalid pageset: more than 1 WPR archive found.: " +
           repr(wpr_archive_names_to_page_urls))
 
   def DidStartBrowser(self, browser):
     self._cpu_metric = cpu.CpuMetric(browser)
     self._cpu_metric.Start(None, None)
 
   def MeasurePage(self, page, tab, results):
-    tab.browser.foreground_tab.WaitForDocumentReadyStateToBeComplete()
+    tab.WaitForDocumentReadyStateToBeComplete()
 
     # Record CPU usage from browser start to when the foreground page is loaded.
     self._cpu_metric.Stop(None, None)
     self._cpu_metric.AddResults(tab, results, 'cpu_utilization')
 
     startup_metric.StartupMetric().AddResults(tab, results)
 
     # TODO(jeremy): Measure time to load - first, last and frontmost tab here.